To achieve large scale acceptance and widespread use of photovoltaic (PV) systems for the generation of electricity and electrical power, new processes must be developed for manufacturing solar cells on a commercial scale with much less cost (directly related to fabrication time) and with much higher cell efficiencies than known fabrication techniques. Today, conventional photovoltaic modules cost about $4.00 per watt (w) and can produce electricity at a rate of about $0.25 per kilowatt hour (kwh). A factor of two in cost reduction is needed to make photovoltaic systems attractive for peak power load applications, and a reduction by a factor of at least three would make photovoltaic systems much more competitive with conventional energy sources for base load utility applications. No photovoltaic material or technology has yet been able to achieve both the cost and efficiency goals simultaneously. Generally, the efficient PV systems are too expensive, and the cheaper PV systems are not efficient enough.
Conventional processes for manufacturing solar cells involve several separate, long, furnace diffusions and oxidations at high temperatures. Moreover, these furnace diffusion and oxidation processes require extensive time periods and meticulous cleaning, prolonged cell processing, and use of large quantities of chemicals, gases, etc. Some researchers have attempted to shorten the fabrication time by using rapid thermal processing (RTP), also known in the art as rapid thermal annealing (RTA), for fabricating silicon solar cells. In general, RTP is a photo-assisted thermal annealing process which utilizes a light radiating source for heating purposes and, in particular, for generating radiant heat.
As examples of RTP research in the field of solar cell fabrication, see the following publications: J. F. Joly, et al., Proceedings of 18th IEEE Photovoltaic Specialists Conference, p. 1756 (IEEE, Las Vegas, 1985); R. Campbell, et al., J. Electrochem. Soc., v. 133, p. 2210 (1986); B. Hartiti, et al., 11th E. C. Photovoltaic Solar Energy Conference, p. 420 (Montreux, Switzerland, 1992); B. Hartiti, et al., Proceedings of 23rd IEEE Photovoltaic Specialists Conference, p. 224 (IEEE, Louisville, 1993); and R. Schindler, et al., Proceedings of 23rd IEEE Photovoltaic Specialists Conference, p. 162 (IEEE, Louisville, 1993). However, as is generally known in the industry and is apparent from the foregoing publications, researchers have had only very limited success in using RTP to produce solar cells of high efficiency.
One reason is that RTP is susceptible to generating electrically-active defects, or traps. In other words, RTP significantly reduces the bulk lifetime .tau. of minority carriers by freezing grown-in or process-induced impurities to form electrically active traps which assist in recombination of photo-generated carriers. These traps undesirably inhibit generation of electricity in a solar cell during excitation by light. For a discussion of the adverse effects of RTP on minority carrier bulk lifetime .tau., see A. Rohatgi, et al., Silicon Processing, ASTM STP, p. 804; and also D. C. Gupta, American Society for Testing and Materials p. 389 (1983).
Because of the foregoing reduction in minority carrier bulk lifetime .tau., one researcher has implemented a further separate annealing process at a high temperature after the RTP step in order to recover minority carrier bulk lifetime .tau.. R. Campbell, et al., J. Electrochem. Soc., v. 133, p. 2210 (1986). However, this post-RTP anneal mitigates the attractiveness of RTP due to moderate cell efficiency and additional cost and time.
Another reason why researchers have had only slight success in using RTP to produce high efficiency solar cells is that it is difficult to obtain desired diffusion profiles with RTP. More specifically, in the manufacture of a solar cell, n-type and/or p-type materials are typically diffused into a silicon substrate to form a diffused region(s) and consequently a p-n junction(s) for generating electricity. However, when using RTP, it is difficult to obtain a desired depth for the diffused region(s), and particularly, shallow depths. Shallow junctions can cut down on undesirable heavy doping effects.